Polycrystalline diamond applied to conventional cutting tools, dressers, dies and other tools and excavation bits and the like is prepared with Co, Ni, Fe or a similar iron group metal, SiC or similar ceramic, or the like used as a sintering aid or a binder. A polycrystalline diamond prepared with carbonate used as a sintering aid is also known (see patent documents 1 and 2).
They are obtained by sintering powdery diamond together with a sintering aid and/or a binder at high pressure and high temperature that allow diamond to thermodynamically stabilize (normally, a pressure of 5-8 GPa and a temperature of 1,300-2,200° C.). The high pressure and high temperature allowing diamond to thermodynamically stabilize, as referred to herein, is for example a temperature-pressure range indicated in non patent document 1, FIG. 1. On the other hand, natural polycrystalline diamonds (e.g., carbonate and ballas) are also known and some of them are applied to excavation bits. However, they significantly vary in material property and their yields are also small. They are thus not positively used industrially.
When Co or a similar iron group metal catalyst is used as a sintering aid to prepare a sintered polycrystalline diamond compact, the sintering aid is contained in the sintered polycrystalline compact, and acts as a catalyst helping diamond to graphitize. The sintered compact is thus inferior in thermal resistance. More specifically, even in an atmosphere of inert gas, the diamond would be graphitized at approximately 700° C.
If the sintering aid as described above is used in a large amount, the difference in thermal expansion between the sintering aid and the diamond facilitates causing micro cracks in the polycrystal. Furthermore, between grains of diamond, Co or other metal of the sintering aid exists as a continuous layer, which is a factor reducing the polycrystal in hardness, strength and other similar mechanical properties. The sintering aid or the binder is contained in the polycrystalline diamond by at least 10% by volume and, as has been aforementioned, acts as a catalyst helping diamond to graphitize. This not the least affects the polycrystalline diamond in hardness, strength and other mechanical properties, and thermal resistance. Accordingly there is a strong demand for a sintered compact of single-phase diamond that does not contain a sintering aid, a binder or the like.
It is also known that the aforementioned sintered polycrystalline diamond compact is increased in thermal resistance by removing metal at grain boundaries of the diamond. While this approach provides thermal resistance increased to approximately 1,200° C., the polycrystal becomes porous and is hence decreased in strength.
When a binder of SiC, which is non metallic material, is used to prepare a sintered polycrystalline diamond compact, the sintered compact has excellent thermal resistance and does not have pores as aforementioned. However, it does not have diamond grains bonded together and is thus small in strength.
When a sintering aid of carbonate is used to prepare a sintered polycrystalline diamond compact, the sintered compact is superior in thermal resistance to a sintered polycrystalline compact prepared with a binder of Co. However, it has a carbonate material at a grain boundary, and would thus be insufficient in mechanical properties.
On the other hand, diamond can be prepared by a method converting graphite, glassy carbon, amorphous carbon or other similar non diamond carbon at ultra high pressure and ultra high temperature directly into diamond without a catalyst or a solvent. This method allows the carbon to be converted from non diamond phase directly to diamond phase and simultaneously sintered to obtain a polycrystal of single-phase diamond.
For example, non patent documents 2-4 disclose that graphite is used as a starting material and subjected to direct conversion at ultra high pressure and ultra high temperature of at least 14-18 GPa and 3,000K, respectively, to provide polycrystalline diamond.
If these methods are used to prepare polycrystalline diamond, however, graphite or a similar, electrically conductive, non diamond carbon is heated by passing an electric current directly therethrough, and it is unavoidable that the polycrystalline diamond has unconverted graphite remaining therein. Furthermore, the methods also provide diamond grains varying in size and tend to provide partially insufficient sintering. This provides unreliable hardness, strength and other mechanical properties, and can only provide a polycrystal in the form of a chip. Furthermore, the methods require ultra high pressure and ultra high temperature exceeding 14 GPa and 3,000K, respectively, and thus entail extremely high production costs and are low in productivity. They are thus inapplicable to cutting tools, bits and the like and have not reached practical utilization.
The present inventors have found in the preparation of polycrystalline diamond by the direct conversion as described above that non diamond carbon or highly pure graphite-like carbon mechanically pulverized in inert gas to be a carbon material which has a microstructure of fine crystal grains of at most tens nm in size or is amorphousized can be used as a material to allow conversion into diamond even at relatively mild, ultra high pressure and ultra high temperature and simultaneously allow crystal grains of diamond having a small grain size of at most tens nm and having a narrow grain size distribution to be firmly bonded together to provide dense polycrystalline diamond formed of substantially 100% diamond, and the present inventors have filed a patent application therefor (see patent document 3).
Furthermore, for example, patent document 4 describes a method heating carbon nanotube to at least 10 GPa and at least 1,600° C. to synthesize fine diamond. However, the carbon nanotube used as a material is expensive and thus contributes to high production cost. Furthermore in the method, the carbon nanotube is pressurized by a diamond anvil transmitting light, and heated by condensed CO2 gas laser light through the anvil. The method in reality cannot produce homogenious polycrystalline diamond of a size applicable to cutting tools and the like.
Accordingly the present inventors have invented a method using high purity graphite as a starting material and subjecting it to direct conversion and sintering by indirect heating at ultra high pressure and ultra high temperature of at least 12 GPa and at least 2,200° C., respectively, to obtain a dense and highly pure polycrystalline diamond (see non patent documents 5 and 6). A polycrystal of single-phase diamond obtained by direct conversion and sintering at ultra high pressure and ultra high temperature with graphite used as material, was applied to a cutting tool for evaluation in performance. It has been found to, on one hand, be significantly superior to a sintered diamond compact containing a binder as conventional, but on the other hand, vary in performance between samples.
In other words, this method provides diamond which on one hand may be significantly hard but on the other hand is insufficient in reproducibility, varies in mechanical property and provides insufficient cutting performance.
A diamond crystal normally provides a first-order Raman spectral line appearing at 1,332.0 cm−1, which is a value of almost defectless and strainless, high quality, synthetic diamond, and it is known that when diamond experiences stress for compression, the value shifts to a higher wave number. It shifts by an amount of approximately 2 cm−1 for 1 GPa, although it depends on how the stress is exerted.
Furthermore, it is also known that a diamond crystal having a diamond grain smaller in size provides spectra weakened and broadened, and a first-order Raman spectral line shifted to a lower wave number (see non patent documents 7 and 8). For example, a diamond grain having as small a size as approximately 1 μm provides a first-order Raman spectral line shifted positionally to a lower wave number by approximately 5 cm−1. More specifically, diamond which is highly pure and devoid of crystal defect and has a grain size of at least 10 μm provides a first-order Raman spectral line appearing at 1,332 cm−1, whereas diamond having as fine a grain size as approximately 1 μm provides a first-order Raman spectral line shifted to 1,327 cm−1.
Patent document 5 discloses a method adding i-carbon or diamond-like carbon to powdery diamond and processing them at high temperature and high pressure in a range allowing diamond to thermodynamically stabilize, to obtain polycrystalline diamond. This method, however, employs powdery diamond having a grain size of at least 1 μm, and furthermore, converts i-carbon into diamond and grows it on a surface of the powdery diamond. As such, the method provides polycrystalline diamond which tends to have unconverted graphite, a void and the like remaining therein (a density of 3.37 g/cm3; approximately 96% of true density of diamond) and also has a hardness of 6,600 kg/mm2, which is a small value for a polycrystal of single-phase diamond.
Furthermore, non diamond type carbon mainly composed of C13 is used as a material to prepare diamond in a method. More specifically, C13 methane is used as a material and chemical vapor deposition (CVD) is employed to obtain C13 polycrystalline diamond. Normally, however, polycrystalline diamond obtained through CVD does not undergo a sintering process. It thus has its grains bonded with small force and also having oriented growth. It is thus insufficient in mechanical property to be applied to cutting tools, anti-wear tools and the like. Furthermore, monocrystalline diamond grown with the CVD-synthesized C13 used as a material is also known (see for example non patent document 9). However, as this diamond is monocrystalline, it is cleavable and has anisotropy in hardness, and is thus inapplicable to a wide range of tools.    Patent Document 1: Japanese Patent Laying-open No. 4-74766    Patent Document 2: Japanese Patent Laying-open No. 4-114966    Patent Document 3: Japanese Patent Laying-open No. 2004-131336    Patent Document 4: Japanese Patent Laying-open No. 2002-66302    Patent Document 5: Japanese Patent Laying-open No. 61-219759    Non-Patent Document 1: F. P. Bundy, et al., Carbon, Vol. 34, No. 2 (1996) 141-153    Non-Patent Document 2: F. P. Bundy, J. Chem. Phys., 38 (1963) 631-643    Non-Patent Document 3: M. Wakatsuki, K. Ichinose, T. Aoki, Jap. J. Appl. Phys., 11 (1972) 578-590    Non-Patent Document 4: S. Naka, K. Horii, Y. Takeda, T. Hanawa, Nature 259 (1976) 38    Non-Patent Document 5: New Diamond and Frontier Carbon Technology, 14 (2004) 313 [T. Irifune, H. Sumiya]    Non-Patent Document 6: SEI Technical Review, 165 (2004) 68 [Sumiya, Irifune]    Non-Patent Document 7: J. Appl. Phys., 72 (1992) 1748 [Y. Namba, E. Heidarpour, M. Nakayama]    Non-Patent Document 8: Appl. Phys. Lett., 62 (1993) 3114 [M. Yoshikawa, Y. Mori, M. Maegawa, G. Katagiri, H. Ishida, A. Ishitani]    Non-Patent Document 9: W. Banholzer et al., New Diamond Science and Technology, 1991, MRS Int. Conf. Proc., pp. 857-862